High-chromium nitrogen containing castable alloy

ABSTRACT

A corrosion and erosion resistant alloy comprising as mandatory elements besides iron, in % by weight, about 31 to about 48 chromium, about 0.01 to about 0.7 nitrogen, about 0.5 to about 30 manganese and about 0.3 to about 2.5 carbon. This abstract is neither intended to define the invention disclosed in this specification nor intended to limit the scope of the invention in any way.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation-in-part of U.S. applicationSer. No. 10/040,357 filed Jan. 9, 2002, the entire disclosure whereof isexpressly incorporated by reference herein.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates generally to the art of alloys and moreparticularly to a high-chromium, nitrogen containing alloy having highcorrosion resistance. The instant invention also relates to ahigh-chromium, nitrogen containing castable alloy, a high-chromiumnitrogen content alloy, and a process for producing the high-chromium,nitrogen containing alloy, and articles prepared from the same.

[0004] 2. Discussion of Background Information

[0005] Equipment used in highly corrosive environments typically is madeof alloys such as stainless steel and other highly alloyed materials.These alloys must be able to withstand the extremely corrosiveenvironments created by chemicals such as concentrated sulfuric acid orconcentrated phosphoric acid. A particularly difficult environment isencountered in the production of phosphate fertilizer. In the digestionof phosphate rock with hot, concentrated sulfuric acid, the equipment isrequired to be resistant to the environment at temperatures up to about100° C. The crude phosphoric acid produced can be extremely corrosiveand contains some residual sulfuric acid. The corrosive effect is oftenincreased by other impurities in the phosphoric acid, particularly byhalogen ions such as chloride and fluoride, which are normally presentin the phosphate rock feedstock that is used in the process. Aparticularly corrosive environment is encountered during theconcentration of the crude phosphoric acid.

[0006] Phosphate rock deposits at various locations in the world varygreatly in chemical composition. The most severe corrosive environmentsare typically encountered in the processing of deposits of phosphaterock which contains a high content of halides, such as chloride orfluoride.

[0007] It is known that increasing the Cr content improves the corrosionresistance of steel. Hi-chromium alloys containing 23-40% Cr, 0.8-2% C,2.5% Si, and up to 5% Mo, have been known since the 1930's. See, forexample, German Patent No. 701,807. U.S. Pat. No. 5,252,149 representsan improvement of this alloy, followed by German Patent Application Nos.195 12 044 and 44 17 261. According to both patents the alloys exhibit ahigh resistance to abrasion and good resistance to corrosion. However,both alloys exhibit poor mechanical properties, especially lowtoughness, brittleness, sensitivity to heat, and low notched impactresistance, thereby limiting their usefulness. It is evident that theirstructures comprise ferrite (Fe-α).

[0008] The ferritic structure comprised in these alloys is inherentlyvery brittle, and the carbide phase embedded in such a brittle phaseresults in a very low toughness, high notch sensitivity, as well assensitivity to heat. Additionally, the ferritic structure issupersaturated with chromium, resulting in a formation of the sigmaphase, which drastically lowers toughness and corrosion resistance.

[0009] U.S. Pat. No. 5,320,801 is directed to alloys having thefollowing composition in % by weight: Cr—27 to 34, Ni+Co—13 to 31,Si—3.2 to 4.5, Cu—2.5 to 4, C—0.7 to 1.6, Mn —0.5 to 1.5, Mo —1 to 4,and Fe—essentially the balance. The alloy of the '801 patent possessesgood toughness, but exhibits very poor hardness and very poor wearresistance and low tensile strength. Its hardness of 208 to 354 HB issimilar to that of CD4MCU stainless steel (260-350 HB), which hasexcellent corrosion resistance, but poor wear resistance. The alloydisclosed in U.S. Pat. No. 5,320,801 is similar to austenitic,high-nickel stainless steels in that it has good toughness, but very lowtensile strength and hardness, as well as poor wear resistance. Thenickel present in corrosion resistant alloys mainly assists instructural stabilization but contributes very little to an improvementin corrosion resistance. Representative examples thereof are thestainless austenitic steels which contain 12-35% Ni and have a corrosionresistance which approaches that of duplex stainless steels whichcontain a low percentage of nickel (4-8%), or that of high-chromiumstainless steels with a Ni content of not more than 4%. The primaryelements of stainless alloys are Cr, Mo and nitrogen, as shown in theExamples below which illustrate how various alloying elements influencethe corrosion resistance of stainless steel. For example, the PittingResistance Equivalent Number (PREN)=% Cr+3.3×% Mo+16×% N illustratesthat nitrogen is an important, very powerful alloying element ofcorrosion resistant alloys.

[0010] One of the main shortcomings of the high-chromium alloys of theprior art is the difficulty in dissolving Cr, Mo and N in the matrixwithout adversely affecting the mechanical properties of the alloy, suchas toughness, tensile strength, brittleness, heat sensitivity andweldability. This difficulty is due to the precipitation of the sigmaphase from alloys which are saturated with chromium and molybdenum.Premature wearing out of pump parts made from the above-mentionedhigh-chromium alloys is a common occurrence. The main contributingfactors in this respect are very low toughness, brittleness and lowendurance. Very often a failure occurs due to a casting which is wornthin in an isolated area where, due to the poor mechanical properties ofthe alloy, a crack developed. Eventually, this leads to the destructionof an otherwise still viable component.

[0011] The mechanisms of corrosion and erosion in acidic environments ofthe alloys of the prior art involve accelerated corrosion due to thecontinuous removal of the passive corrosion resistant layer by particlescontained in corrosive fluids. This is most evident in alloys whichcontain a higher content of Cr and Mo, where a significant amount ofsigma phase is unavoidable and the metal matrix possesses very poortoughness properties. In order to restore the passive layer, it isnecessary to use as high as possible concentrations of Cr and Mo.

[0012] An increase in the Cr/C, or (Cr+Mo)/C ratio increases thecorrosion resistance up to the critical point, after which the formationof the sigma phase begins, which drastically reduces the toughness andlowers the corrosion resistance of the alloy by depleting the Cr in thevicinity of the sigma phase precipitates.

[0013] The present invention provides an increase in the ratio(Cr+N)/(C−N), or (Cr+Mo+N)/C and (Cr+Mo+N+B)/(C−N) by reducing thecarbon content in the matrix and introducing nitrogen as a powerfuladditional alloying element to the high-chromium alloys, where it ispresent in a high concentration in solid solution.

[0014] Nitrogen, like carbon, forms interstitial solids withbody-centered-cubic (bcc) α-iron, and face-centered-cubic (fcc) γ-iron.The size of the nitrogen atom is smaller than that of the carbon atom,wherefor the nitrogen atom can occupy the interstitial sites in the α-aswell as in the γ-phases more easily than the carbon atom.

[0015] The maximum solubility of nitrogen in Fe-α and Fe-γ is severaltimes higher than that of carbon at the same temperature, which leads toa substantial expansion and distortion of elementary lattices. Nitrogenhas a solid solution hardening and strengthening effect that is muchgreater than that of carbon, while at the same time maintaining a higherlevel of toughness.

[0016] The solubility limits of nitrogen in the prior art high-chromiumalloys are a very low, 0.15% of N at the most. This limit is dictated bythe inherently low physico-chemical solubility of nitrogen and carbon(0.02 to 0.08% max. for C+N) in the Fe-α structure, which constitutes upto a maximum of 40% of the alloys of German Patent Application Nos. 4417 261 or 195 12 044 as well as by a low Mn content of ≦1.5

[0017] The addition of nitrogen is the most effective means forimproving the mechanical properties of austenitic high-chromium alloyswithout deleteriously affecting the ductility and corrosion resistancethereof. It has now been found that if Mn and/or Mo are present inconsiderable amounts in high-chromium alloys, nitrogen can be fullyeffective as an anti-corrosive agent, and may have a wide range ofpositive effects on the mechanical properties of a casting such as,e.g., increased tensile strength, hardness and toughness, without givingrise to a loss in ductility. In particular, under these conditionsnitrogen dissolves in the solid state two to four times better than inany other high-chromium alloy of in the prior art. Similarly, inhigh-manganese stainless steels, which dissolve up to 0.8% nitrogen, andeven up to 1% under high nitrogen partial pressure, the tensile strengthand the hardness may be increased by a factor of two to four, with asgood a ductility as for the same steel without nitrogen.

SUMMARY OF THE INVENTION

[0018] The present invention provides a corrosion and erosion resistantalloy which comprises, in % by weight, about 31 to about 48 chromium,about 0.01 to about 0.7 nitrogen, about 0.5 to about 30 manganese, about0.3 to about 2.5 carbon, 0 to about 5 boron, 0 to about 6 molybdenum, 0to about 5 silicon, 0 to about 8 copper, 0 to about 4 cobalt and 0 toabout 25 nickel plus cobalt. This alloy further comprises 0 to about 2%of each of zirconium, vanadium, cerium, titanium, tantalum, tungsten,niobium, aluminum, calcium and rare earth elements, the balancecomprising iron and inevitable impurities. It has a microstructure whichcomprises chromium carbides, nitrides and optionally borides in anaustenitic matrix, which matrix has a face centered cubic structure andis supersaturated with nitrogen in solid solution form. The compositionof the alloy satisfies the relationship:$\frac{{\% \quad {Ni}} + {\% \quad {Co}} + {0.5\left( {{\% \quad {Mn}} + {\% \quad {Cu}}} \right)} + {30\left( {{\% \quad N} + {\% \quad C}} \right)} + {5x\quad \% \quad B}}{{\% \quad {Cr}} + {\% \quad {Mo}} + {\% \quad {Si}} + {1.5\left( {{\% \quad {Ti}} + {\% \quad {Ta}} + {\% \quad V} + {\% \quad {Nb}} + {\% \quad {Ce}} + {\% \quad {Al}}} \right)}} \geq {1.5.}$

[0019] In one aspect, the alloy may comprise molybdenum, silicon, boron,copper and/or (nickel plus cobalt), each in an amount of at least about0.01% by weight.

[0020] In another aspect, the alloy may have a PREN of from 58 to 66and/or the matrix may comprise from about 0.25% to about. 0.45% ofnitrogen in solid solution form.

[0021] In yet another aspect, the alloy may comprise at least about 32%by weight of chromium.

[0022] In a still further aspect, the alloy may comprise about 32 toabout 34 chromium, about 0.35 to about 0.45 nitrogen, about 6 to about 9manganese, about 0.5 to about 2.5 carbon, 0 to about 4.5 boron, 0 toabout 5 molybdenum, 0 to about 3 silicon, 0 to about 4 copper, 0 toabout 4 cobalt and 0 to about 4 nickel plus cobalt. This alloy maycomprise about 2 to about 5 molybdenum, about 0.5 to about 3 silicon,about 0.7 to about 4 copper and/or about 1.5 to about 4 nickel pluscobalt. For example, about 2 to about 4 molybdenum, about 0.5 to about 2silicon, about 0.7 to about 3 copper and about 1.5 to about 3 nickelplus cobalt may be present in this alloy. Moreover, the alloy maycomprise at least about 0.01% by weight of boron.

[0023] In a still further aspect, the alloy of the present invention maycomprise, in % by weight, about 35 to about 40 chromium, about 0.4 toabout 0.6 nitrogen, about 4.5 to about 15 manganese, about 0.8 to about1.6 carbon, 0 to about 5 boron, 0 to about 5 molybdenum, 0 to about 3silicon, 0 to about 6 copper, 0 to about 4 cobalt and 0 to about 13nickel plus cobalt. This alloy may comprise, for example, about 2 toabout 4 molybdenum, about 0.5 to about 2 silicon, about 1 to about 4copper and/or about 4 to about 13 nickel plus cobalt, e.g., it maycomprise about 0.9 to about 1.6 carbon, about 5 to about 13 manganese,about 2 to about 4 molybdenum, 0 to about 4.5 boron, about 0.5 to about1.5 silicon, about 1 to about 3 copper, about 0.01 to about 4 cobalt,and about 4 to about 12.5 nickel plus cobalt, or it may comprise about 1to about 1.55 carbon, about 5 to about 12 manganese, about 2 to about3.5 molybdenum, 0 to about 4 boron, about 0.6 to about 1.2 silicon,about 1 to about 2.5 copper, about 0.02 to about 4 cobalt and about 4 toabout 12 nickel plus cobalt. This alloy may have a PREN is from 58 to 66and/or the matrix thereof may comprise from about 0.25% to about 0.45%by weight of nitrogen in solid solution form.

[0024] In yet another aspect of the alloy of the present invention, thealloy may comprise, in % by weight, about 41 to about 48 chromium, about0.45 to about 0.7 nitrogen, about 6 to about 30 manganese, about 0.9 toabout 1.5 carbon, 0 to about 3.5 boron, 0 to about 4 molybdenum, 0 toabout 3 silicon, 0 to about 8 copper and 0 to about 25 nickel pluscobalt. This alloy may comprise, for example, molybdenum, silicon,boron, copper and/or (nickel plus cobalt), each in an amount of at leastabout 0.01% by weight. Particularly, this alloy may comprise about 1 toabout 4 molybdenum, about 0.5 to about 3 silicon, about 1 to about 8copper and about 10 to about 25 nickel plus cobalt. Also, it may have aPREN of from 51 to 72 and/or the matrix thereof may comprise from about0.25% to about 0.45% by weight of nitrogen in solid solution form.

[0025] The present invention also provides a casting of the alloy of thepresent invention, including the various aspects thereof. For example,the casting may be a casing, impeller, suction liner, pipe, nozzle,agitator or a valve blade.

[0026] The present invention provides a high-chromium alloy and, morespecifically, a corrosion and erosion resistant high-chromium, nitrogencontaining castable alloy. The alloy of the present invention may beused, for example, for the manufacture by casting of slurry pump parts,such as casings, impellers, suction liners, pipes, nozzles, agitators,valve blades, in particular, for casting parts will be exposed to highlycorrosive fluids and abrasive slurries. A typical application for suchparts is in the wet processing of phosphoric acid. Industrial phosphoricacid solutions are chemically complex, containing sulfuric acid,hydrofluoric acid, hydrochloric acid, chlorides, fluorides and gypsum,all highly depassivating species which are highly detrimental to theparts exposed thereto. Another application for such parts is in powerplant scrubbers i.e., flue gas desulfurization processes where the partsare exposed to sulfuric components and gypsum.

[0027] One of the objects of the present invention is to provide amaterial with high resistance to chloride environments, which at thesame time exhibits extraordinary properties in acidic and basicenvironments, combined with good mechanical properties and highstructural stability. This combination can be very useful inapplications in, for example, the chemical industry, where problemsexist with respect to corrosion caused by acids, and a contamination ofthe acids with chlorides amplifies the corrosive effect. Theseproperties of the alloy in combination with a high strength lead toadvantageous design solutions from an economic point of view. Currentlyavailable materials with good properties in acidic environments includesteels with high contents of Ni, which makes these materials veryexpensive. Another disadvantage of austenitic steels is that theyusually exhibit a very low strength.

[0028] It has been found that the maximum solubility of nitrogen in asolid solution of the FeCr—Mn alloys of the present invention is about0.013 to about 0.0155% N with 1% of Cr and a minimum of 6% of Mn and aminimum of 2% of Mo as the best enhancement.

[0029] Nitrogen has a much lower affinity toward Cr than carbon. Theabove-mentioned properties of nitrogen in high-chromium-manganese alloyscause the carbon in these alloys to be transformed into the carbidephase, forming hard eutectic chromium carbides, with the surplus carbonbeing dissolved in the matrix together with nitrogen.

[0030] Nitrogen introduced in a high concentration in solid solution hasa much stronger effect than carbon on the retardation of the formationthe sigma phase, thereby allowing larger quantities of Cr and Mo to bedissolved in the Fe—Cr—Mn alloys to enhance passivation.

[0031] Nitrogen generally improves corrosion resistance, particularly inchloride containing media. In stainless steels its effectiveness hasbeen tested and expressed by the PREN value (Pitting ResistanceEquivalent Number)=% Cr+3.3×% Mo+16×% N. The higher the level of thepassivating elements (Cr, Mo, N), the higher the resistance tocorrosion/erosion.

[0032] Additionally, boron reacts with many elements in the PeriodicTable to form a wide variety of compounds. The strong covalent bonds ofmost borides are responsible for their high melting points, corrosionresistance and hardness values. The chemical resistance of borides issuperior to that of most of their nitride and carbide counterparts.Because of the larger atomic size of B≈0.91 Å, compared to C≈0.77 Å andN≈0.71 Å, interstitial substitution of boron in an undistortedoctahedral site is rare, resulting primarily in boron-boron bonding, forborides M_(n)B_(m) (NiB, CoB, MnB, FeB, CrB)

[0033] In addition, nickel, manganese and iron react strongly with boronand form very hard compounds, much harder than the correspondingnitrides or carbides. For extremely abrasive and corrosive applicationsboron is preferably employed in concentrations of up to about 5% B, witha carbon content of from about 0.3% to about 1.2% and a nitrogen contentof form about 0.4% to about 0.6%.

[0034] Overall superior results are realized according to the presentinvention by the novel microstructure, with a highly corrosion-resistantmatrix, preferably austenitic, of face centered cubic crystal structure,and supersaturated by nitrogen in solid solution form. This matrix isvery hard, tough, non-brittle and has carbides and nitrides (andoptionally borides) embedded therein, which additionally imparts highwear resistance to the matrix.

[0035] In practicing the instant invention, it is preferred for thematrix to contain high levels of Cr, Mo and N in a solid solution,without Cr, or Mo, being trapped by sigma phase precipitates. It isdesired that the constituting elements of the alloys of the presentinvention satisfy the following relationship, which is a measure of theausteniticity of the present alloys:$\frac{\left. {{\% \quad {Ni}} + {\% \quad {Co}} + {0.5\left( {{\% \quad {Mn}} + {\% \quad {Cu}}} \right)} + {30\left( {{\% \quad N} + {\% \quad C}} \right)} + {5x\quad \% \quad B}} \right)}{{\% \quad {Cr}} + {\% \quad {Mo}} + {\% \quad {Si}} + {1.5\left( {{Ti} + {Ta} + V + \quad {Nb} + {Ce} + {Al}} \right)}} \geq 1.5$

[0036] Due to the addition of the austenite-formers nickel and cobalt ina concentration range of about 0.01% to about 25 wt.-%, it is possibleto control the ratio of the ferrite and austenite phases in the matrixin a defined manner. The normally extremely high brittleness of chilledcasting types with high carbon contents and a carbide lattice in aferritic matrix is avoided by the predominant deposition of the chromiumcarbides in the only phase present, i.e., the austenitic phase. Sincethe austenitic phase, unlike the ferrite phase, is not embrittled bysegregation of intermetallic phases or by segregation processes, thedanger of fractures due to stresses between the carbides and the matrixis not as great as it is in the case of a purely ferritic orferritic-austenitic matrix.

[0037] A molybdenum content within the range of from about 0.01% toabout 6 weight %, preferably from about 2% to about 4 weight %, andespecially about 2% to about 3 weight %, increases the corrosionresistance, especially in chloride-containing, acidic media.

[0038] Also, by varying the alloy components carbon and chromium withinthe range of from about 0.3% to about 2.5 weight % for carbon and fromabout 31% to about 48 weight % for chromium, the corrosion resistanceand wear resistance of the material of the invention can be adjusted tocorrespond to a prescribed specification profile.

[0039] The corrosion resistant high-chromium, nitrogen containingaustenitic alloy of the present invention has excellent high temperaturestrength and is suitable as construction material for boilers, chemicalplant reactors and other equipment which is regularly exposed to hightemperatures and/or corrosive environments. The present invention alsoprovides a metal casting material the wear resistance of whichcorresponds approximately to that of materials of common commercialtypes of white iron, and which additionally exhibits high corrosionresistance in aggressive media. In addition to high corrosion and wearresistance, the alloy according to the invention has good castingcharacteristics. Accordingly, it can be produced in conventionalhigh-grade steel foundries. The casting material also has good workingcharacteristics. These advantageous characteristics of the alloy of thepresent invention may be attributed primarily to a chromium content ofabout 31 to about 48 wt. %, a carbon content of about 0.3 to about 2.5wt. %, and a nitrogen content of about 0.01 to about 0.7 wt. %, whichaffords a sufficiently high volume proportion of carbides and nitrides.The high-chromium content decreases the chromium depletion of thematrix. Compared to the known types of castings previously utilized inapplications involving hydroabrasive wear, the material according to thepresent invention shows a much better combination of corrosionresistance and wear resistance. The present invention is also directedto an air-meltable, castable, workable alloy which exhibits resistanceto the corrosive action of acids such as sulfuric acid and phosphoricacid over a wide range of acid strengths.

[0040] The high-chromium, nitrogen containing alloy composition of thepresent invention is also highly responsive to a cryogenic hardeningprocess, thereby becoming super-hard. When hardened by the cryogenictreatment, the composition possesses higher abrasion resistance, greaterhardness, and a durable matrix without the usual precipitation ofsecondary carbides.

[0041] The alloys of the invention may be prepared by conventionalmethods of melting, and no special conditions, such as, e.g., controlledatmosphere, special furnace linings, protective slags or special moldingmaterials are required.

[0042] In the treatment process of the present invention, thehigh-chromium, nitrogen containing castable alloy has many of thealloying elements entirely distributed in the austenitic phase or itstransformation products, when subjected to sub-zero treatment of atleast −100° F., preferably −100° F. to −300° F., attain much greaterhardening than that achieved through conventional high temperaturetreatments.

[0043] Generally, the high-chromium, nitrogen containing alloys of thisinvention are made by preparing a molten metal mass of all the requiredelements in the presence of air or additional nitrogen, pouring castingstherefrom, cooling of the castings, and subjecting the castings to acryogenic cooling treatment to produce the desired hardness. The surfaceof the casting may be cleaned and finished, either before or aftercryogenic cooling. In more detail, the preferred process involves thefollowing steps:

[0044] (1) mixing the necessary components to be fed to the furnace;

[0045] (2) melting the mixture in the furnace to form a pourable moltenmetal;

[0046] (3) pouring the molten metal composition into an appropriatemold;

[0047] (4) allowing the mold and the casting therein to cool slowly toroom temperature under ambient conditions;

[0048] (5) cleaning and finishing the surface of the casting, forexample, by grinding or the like to smooth the surface; and,

[0049] (6) immersing the finished casting in a cryogenic cooling mediumat a temperature of −100° F. to −300° F. for a time sufficient to reachthe desired hardness.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

[0050] The particulars shown herein are by way of example and forpurposes of illustrative discussion of the embodiments of the presentinvention only and are presented in the cause of providing what isbelieved to be the most useful and readily understood description of theprinciples and conceptual aspects of the present invention. In thisregard, no attempt is made to show structural details of the presentinvention in more detail than is necessary for the fundamentalunderstanding of the present invention, the description making apparentto those skilled in the art how the several forms of the presentinvention may be embodied in practice.

[0051] Several mechanical tests as described below were conducted whichincluded the following measurements:

[0052] Tensile Strength—(Ksi)

[0053] Deflection—(mm), 30.5 mm diameter cast bar, 300 mm span.

[0054] Impact Resistance—(J), IZOD test, unnotched 30.5 mm diameter bar,struck 76 mm above support.

[0055] Hardness—(BHN): Brinell test, 3000 kg load on 10 mm tungstencarbide ball.

[0056] For the testing, alloys according to the present invention werecompared to alloys of the prior art alloys, and stainless steel as areference.

[0057] The specific alloys tested were as follows:

[0058] Preferred alloys (composition in wt. %) of U.S. Pat. No.5,252,149: 1 2 3 Cr 36.6 Cr 38.2 Cr 39.3 C 1.9 C 2.06 C 2.02 Mn 1.2 Mn1.5 Mn 1.1 Si 1.5 Si 1.4 Si 1.5 Ni 2 Mo 1.2 Mo 1.8 Cu 1 Ni 1.2 Ni 1.6Balance - Fe Cu 1.2 Cu 1.6 plus inevitable impurities Balance - FeBalance - Fe plus plus inevitable inevitable impurities impurities

[0059] Preferred alloys (composition in wt. %) of U.S. Pat. No.5,320,801: 4 5 6 Cr 29.8 Cr 32.7 Cr 34.8 Ni + Co 17.2 Ni + Co 26.5 Ni +Co 34.5 Si 3.4 Si 3.2 Si 3.5 Cu 1.9 Cu 3.1 Cu 3.8 C 1.65 C 1.28 C 1.26Mn 1.1 Mn 1.5 Mn 1.6 Mo 0.9 Mo 1.8 Mo 2.2 Balance - Fe Balance - FeBalance - Fe plus plus plus inevitable inevitable inevitable impuritiesimpurities impurities

[0060] Alloys according to the present invention (composition in wt. %):7 8 8B 9 9a* 9b* Cr 35.8 Cr 37.3 Cr 37.9 Cr 38.3 Cr 39.1 Cr 33.4 N 0.42N 0.48 N 0.4 N 0.52 N 0.56 N 0.41 Mn 6.1 Mn 9.8 Mn 5.2 Mn 11.1 Mn 8.2 Mn5.1 C 1.26 C 1.33 C 1.33 C 1.41 C 1.55 C 2.2 B 0.2 B 0.15 B 3.8 B 0.1 B0 B 0 Mo 3 Mo 2.6 Mo 2.6 Mo 2.2 Mo 2.1 Mo 2.7 Si 0.9 Si 0.8 Si 1 Si 0.7Si 0.7 Si 0.94 Cu 1.5 Cu 1.7 Cu 1 Cu 1.9 Cu 1.5 Cu 0.8 Co 2.1 Co 0.6 Co0.5 Co 4 Co 0.02 Co 0.004 Ni 3.25 Ni 3.6 Ni 8.2 Ni 0.2 Ni 11.0 Ni 1.8Balance - Fe Balance - Fe Balance - Fe Balance - Fe Balance - FeBalance - Fe plus plus plus plus plus plus inevitable inevitableinevitable inevitable inevitable inevitable impurities impuritiesimpurities impurities impurities impurities

[0061] Alloys of German Patent Application Nos. 19512044 and 4417261(composition in wt. %): 10 11 12 Cr 38.8 Cr 43 Cr 44 Ni 5 Ni 8 Ni 10 Mo2 Mo 3 Mo 3.5 Cu 2 Cu 2.5 Cu 2.1 N 0.19 N 0.09 N 0.15 Si 1 Si 1.5 Si 1.5Mn 1 Mn 1.2 Mn 1.1 C 1.6 C 1.7 C 1.6 V 1.2 Balance - Fe Balance - FeBalance - Fe plus inevitable plus inevitable plus inevitable impuritiesimpurities impurities

[0062] Stainless Steel Alloy (composition in wt. %): 20Cb3 Cd-4MCu+N317L Cr 20 Cr 26.5 Cr 18 Ni 37.5 Ni 5.5 Ni 11 Mo 3 Mo 2.5 Mo 3.1 Cu 3 Cu2.9 C Min. Nb 0.4 N 0.23 C Min C Min Balance - Fe plus Balance - Fe plusBalance - Fe plus inevitable inevitable inevitable impurities impuritiesimpurities

[0063] TABLE 1 Sample No. Tensile U.S. Pat. Strength Deflection ImpactHardness No. (Ksi) Elongation % (mm) (J) (BHN) Comments 5,252,149  1 610 2/3 12 19 450 as cast  2 64 0 1.3/1.9 11 18 460  3 58 0 0.9-1.9 10 16490 Heat treatment at 1450° F. for 3 hrs 5,320,801  4 53 0 8-11 22-26360 Sample:  5 54 0.3-0.6 9-13 26-34 330 Hardened at 1400° F. for 4 hrs 6 48 0.3-0.5 8-13 22 31 320 Hardened at 1400° F. for 4 hrs Presentinvention  7 95 0.5-1.1 14-18 48-59 512 Cryogenic C hardened at - 300°F.  8 111 0.4-1.0 10-16 41-49 450 Heat Treated  8B 109 0 8-12 30-36 530as cast  9 95 0.3-0.6 9-12 36-47 490 as cast German Patent Appls.4417261, 19512044 10 68 0 1.5-2.2 11-16 500 Heat treatment at 1800° F.for 2 hrs 11 65 0 1 2.0 10-15 450 12 64 0 0.6 1.6 8-14 490

[0064] The alloys 1, 2, 3, 10, 11 and 12 of the prior art have aneutectic microstructure where the matrices are essentially ferritic(Fe-α).

[0065] The alloys according to German Patent Application Nos. 4417261 or19512044, identified as 10, 11 and 12, can have up to 40% or Fe-α phasein the matrix. The Fe-α phase in the high-chromium alloys inherentlyposses very low toughness because of the very low solubility of carbonand nitrogen in the Fe-α. Even a small, limited addition of nitrogen hasa detrimental effect on the toughness, deflection and heat sensitivity,making the alloy more brittle.

[0066] Alloys 4, 5 and 6 of U.S. Pat. No. 5,320,801 are chromiumhigh-nickel alloys with an austenitic microstructure. These high-nickelalloys inherently possess a very low tensile strength, a very lowhardness, as cast above 200 HB, and after hardening from the range of300 HB, they lose their toughness and corrosion resistance.

[0067] As can be appreciated from Table 1 above, alloys 7, 8 and 9 ofthe present invention possess the following properties in comparison toprior art alloys:

[0068] 2 to 3 times greater toughness

[0069] 1.6 to 2.3 times higher tensile strength

[0070] Very high as cast hardness after cryogenic hardening

[0071] Measurable elongation or malleability

[0072] Excellent deflection

[0073] 1.5 to 2.5 higher max. hydraulic pressure vessel test.

[0074] Low heat sensitivity

[0075] Good machinability, especially threadability, which is very poorin prior art alloys

[0076] Best castability with melting and pouring temp.−150° F. lower

[0077] The alloys of the prior art as well as the alloys of the presentinvention were subjected to corrosion test to show the superiority ofthe alloys of the instant invention:

[0078] The corrosion tests were conducted in synthetic P₂O₅ acid at 80°C., with a chloride content of from 1000 to 3000 ppm. Agitated, 96 hrtest. (mmy). The results of the corrosion tests are summarized in Table2. TABLE 2 PREN = Sample No. Hardness Chloride Corrosion % Cr + 3.3 × %Patent No. (BHN) Content (PPM) Rate (mmy) Mo + 16 × % N U.S. Pat. No.5,320,801 260 1000 17 PREN₅ = 38 5 2000 28 As cast 3000 56 5 330 1000 23Hardened 2000 36 At 1400° F./4 hr 3000 65 U.S. Pat. No. 5,252,149 4601000 15 PREN₂ = 42 2 2000 23 as cast 3000 49 Present 450 1000 8 PREN₈ =53 Invention 2000 11 8 3000 16 As Cast Stainless Steel 180 1000 13 PREN= 30 20Cb-3 2000 14 (20Cb-3) 3000 32 Stainless Steel 280 1000 11 PREN =38 CD-4MCuN 2000 15 3000 19 CD-4MCuN 330 1000 17 Hardened 2000 28 300045 Stainless Steel 185 1000 0.68 PREN = 38 317L 2000 1.1 (317L) 3000

[0079] The following conclusions can be drawn from Table 2:

[0080] The high-chromium alloy No. 5 of U.S. Pat. No. 5,320,801containing 26% Nickel has a lower corrosion resistance than alloy No. 2of U.S. Pat. No. 5,252,149, which has a nickel content of only 1%.

[0081] The same conclusion applies to the stainless steel alloy 20Cb3,in which the Ni content is 37%. The alloy CD4MCuN contains only 5% Ni.The main function of Ni in corrosion resistant alloys is as a structuralcomponent.

[0082] The high-chromium, nitrogen containing alloy No. 8 of the presentinvention contains only 3.6% Ni, but 0.48% nitrogen, which is a verypowerful corrosion inhibitor. Nitrogen interacts with the chlorides andsomehow buffers their detrimental effect on the alloy. Alloy No. 8according to the present invention with the higher PREN=53, has a 2 to 3times better corrosion resistance than the prior art alloys No. 5 andNo. 2. Alloy No. 8 of the present invention which contains high levelsof Cr, Mo and a high concentration of nitrogen, possesses the bestcorrosion resistance in acidic environments which contain high levels ofchlorides.

[0083] Prior art alloys and the alloys of the present invention werealso subjected to corrosion/erosion tests as described below.

Corrosion/Erosion Test

[0084] The corrosion erosion tests were done using 30% by weight ofalumina (80 microns) suspended in 28% P₂O₅ synthetic acid, 1.5% H₂SO₄,0.05% hydrofluoric acid plus 1000 ppm Cl, temperature 800° C., rotation650 RPM, duration 12 hr. Mass loss (mg). The results of theerosion/corrosion testing are shown in Table 3 below. TABLE 3 PREN =Hardness Weight Loss CR % + 3.3 × Sample No. BMN (mg) Mo % + 16 × N U.S.Pat. No. 5,320,801 260 306.6 PREN (5) = 38 5 as cast 5 330 282.6 agehardened at 1400° F./4 hr. Present invention 530 96.3 PREN (8B) = 53 8 -B 450 123.3 8 as cast 8 anneal/S solution 450 125.1 PREN (8) = 53 at2000° F./4 hr. Stainless Steel 280 426 PREN = 38 CD4McuN (CD-4mcUn)solution annealed CD-4MCuN 330 328.2 age hardened 20cb - 3 180 660.3PREN = 30 solution annealed (20Cb-3)

[0085] The slurry corrosion/erosion tests indicate that alloy 20Cb-3which has the lowest hardness shows the highest mass loss. Prior artalloy No. 5 has a low hardness, comparable to the hardness of thereference stainless steel CD-4MCuN.

[0086] The loss of mass of alloy No. 5 of U.S. Pat. No. 5,320,801 is 50%less than that of the stainless steel alloy Cd4MCuN. With alloy sampleNo. 8 according to the present invention the loss of mass is 245% lessthan that of the reference alloy Cd4MCuN. Alloy No. 8 with the highestPREN factor=53, possesses the highest corrosion/erosion resistance,i.e., about 3.5 times better than that of the reference alloy CD4MCuNand 2.3 times better than that of alloy No. 5 according to U.S. Pat. No.5,320,801.

[0087] Alloy No. 8B with boron according to the present invention withthe highest hardness and PREN=53 possesses the highest corrosion/erosionresistance, i.e., about 4.4 times better than that of the referencealloy CD-4MCuN and 2.9 times better than that of alloy No. 5 accordingto U.S. Pat. No. 5,320,801.

[0088] Any conventional or under nitrogen partial pressure castingtechnology may be used to produce the alloys of the present invention.

[0089] It is preferred that the alloys are formed by a conventionalcasting technology and then are heat-treated at a temperature in therange of 1800° to 2000° F., followed by air cooling.

[0090] The most preferred hardening method for the alloy of the presentinvention is by cryogenic treatment: cooling to at least from −100° F.to −300° F., and maintaining at these temperatures for a time of onehour per one inch of casting wall thickness.

[0091] The cryogenic tempering process may be performed with equipmentand machinery which is conventional in the thermal cycling treatmentfield. First, the articles-under-treatment are placed in a treatmentchamber which is connected to a supply of cryogenic fluid, such asliquid nitrogen or a similar low temperature fluid. Exposure of thechamber to the influence of the cryogenic fluid lowers the temperatureuntil the desired level is reached. In the case of liquid nitrogen, thisis about −300° F. (i.e., 300° F. below zero).

[0092] It is noted that the foregoing examples have been provided merelyfor the purpose of explanation and are in no way to be construed aslimiting of the present invention. While the present invention has beendescribed with reference to exemplary embodiments, it is understood thatthe words which have been used herein are words of description andillustration, rather than words of limitation. Changes may be made,within the purview of the appended claims, as presently stated and asamended, without departing from the scope and spirit of the presentinvention in its aspects. Although the present invention has beendescribed herein with reference to particular means, materials andembodiments, the present invention is not intended to be limited to theparticulars disclosed herein; rather, the present invention extends toall functionally equivalent structures, methods and uses, such as arewithin the scope of the appended claims.

What is claimed is:
 1. A corrosion and erosion resistant alloycomprising, in % by weight: from about 31 to about 48 chromium fromabout 0.01 to about 0.7 nitrogen from about 0.5 to about 30 manganesefrom about 0.3 to about 2.5 carbon from 0 to about 5 boron from 0 toabout 6 molybdenum from 0 to about 5 silicon from 0 to about 8 copperfrom 0 to about 4 cobalt from 0 to about 25 nickel plus cobalt, saidalloy further comprising from 0 to about 2% of each of zirconium,vanadium, cerium, titanium, tantalum, tungsten, niobium, aluminum,calcium and rare earth elements, the balance comprising iron andinevitable impurities, said alloy having a microstructure comprisingchromium carbides, nitrides and optionally borides in an austeniticmatrix, said matrix having a face centered cubic crystal structure andbeing supersaturated with nitrogen in solid solution form, thecomposition of the alloy satisfying the relationship:$\frac{{\% \quad {Ni}} + {\% \quad {Co}} + {0.5\left( {{\% \quad {Mn}} + {\% \quad {Cu}}} \right)} + {30\left( {{\% \quad N} + {\% \quad C}} \right)} + {5x\quad \% \quad B}}{\begin{matrix}{{\% \quad {Cr}} + {\% \quad {Mo}} + {\% \quad {Si}} +} \\{1.5\left( {{\% \quad {Ti}} + {\% \quad {Ta}} + {\% \quad V} + {\% \quad {Nb}} + {\% \quad {Ce}} + {\% \quad {Al}}} \right)}\end{matrix}} \geq {1.5.}$


2. The alloy of claim 1, wherein the alloy comprises of at least one ofmolybdenum, silicon, boron, copper and (nickel plus cobalt), each in anamount of at least about 0.01% by weight.
 3. The alloy of claim 1,wherein the alloy comprises at least about 32% by weight of chromium. 4.The alloy of claim 3, wherein the alloy comprises of at least one ofmolybdenum, silicon, boron, copper and (nickel plus cobalt), each in anamount of at least about 0.01% by weight.
 5. A corrosion and erosionresistant alloy comprising, in % by weight: from about 32 to about 34chromium from about 0.35 to about 0.45 nitrogen from about 6 to about 9manganese from about 0.5 to about 2.5 carbon from 0 to about 4.5 boronfrom 0 to about 5 molybdenum from 0 to about 3 silicon from 0 to about 4copper from 0 to about 4 cobalt from 0 to about 4 nickel plus cobalt,said alloy further comprising 0 to about 2% of each of zirconium,vanadium, cerium, titanium, tantalum, tungsten, niobium, aluminum,calcium and rare earth elements, the balance comprising iron andinevitable impurities, said alloy having a microstructure comprisingchromium carbides, nitrides and optionally borides in an austeniticmatrix, said matrix having a face centered cubic crystal structure andbeing supersaturated with nitrogen in solid solution form, thecomposition of the alloy satisfying the relationship:$\frac{{\% \quad {Ni}} + {\% \quad {Co}} + {0.5\left( {{\% \quad {Mn}} + {\% \quad {Cu}}} \right)} + {30\left( {{\% \quad N} + {\% \quad C}} \right)} + {5x\quad \% \quad B}}{\begin{matrix}{{\% \quad {Cr}} + {\% \quad {Mo}} + {\% \quad {Si}} +} \\{1.5\left( {{\% \quad {Ti}} + {\% \quad {Ta}} + {\% \quad V} + {\% \quad {Nb}} + {\% \quad {Ce}} + {\% \quad {Al}}} \right)}\end{matrix}} \geq {1.5.}$


6. The alloy of claim 5, wherein the alloy comprises, in % by weight,one or more of the following: from about 2 to about 5 molybdenum fromabout 0.5 to about 3 silicon from about 0.7 to about 4 copper from about1.5 to about 4 nickel plus cobalt.
 7. The alloy of claim 6, wherein thealloy comprises, in % by weight: from about 2 to about 4 molybdenum fromabout 0.5 to about 2 silicon from about 0.7 to about 3 copper from about1.5 to about 3 nickel plus cobalt.
 8. The alloy of claim 6, wherein thealloy comprises at least about 0.01% by weight of boron.
 9. A corrosionand erosion resistant alloy comprising, in % by weight: from about 35 toabout 40 chromium from about 0.4 to about 0.6 nitrogen from about 4.5 toabout 15 manganese from about 0.8 to about 1.6 carbon from 0 to about 5boron from 0 to about 5 molybdenum from 0 to about 3 silicon from 0 toabout 6 copper from 0 to about 4 cobalt from 0 to about 13 nickel pluscobalt, said alloy further comprising from 0 to about 2% of each ofzirconium, vanadium, cerium, titanium, tantalum, tungsten, niobium,aluminum, calcium and rare earth elements, the balance comprising ironand inevitable impurities, said alloy having a microstructure comprisingchromium carbides, nitrides and optionally borides in an austeniticmatrix, said matrix having a face centered cubic crystal structure andbeing supersaturated with nitrogen in solid solution form, thecomposition of the alloy satisfying the relationship:$\frac{{\% \quad {Ni}} + {\% \quad {Co}} + {0.5\left( {{\% \quad {Mn}} + {\% \quad {Cu}}} \right)} + {30\left( {{\% N} + {\% C}} \right)} + {5x\% B}}{\begin{matrix}{{\% \quad {Cr}} + {\% \quad {Mo}} + {\% \quad {Si}} +} \\{1.5\left( {{\% \quad {Ti}} + {\% \quad {Ta}} + {\% \quad V} + {\% \quad {Nb}} + {\% \quad {Ce}} + {\% \quad {Al}}} \right)}\end{matrix}} \geq {1.5.}$


10. The alloy of claim 9, wherein the alloy comprises, in % by weight,one or more of the following: from about 2 to about 4 molybdenum fromabout 0.5 to about 2 silicon from about 1 to about 4 copper from about 4to about 13 nickel plus cobalt.
 11. The alloy of claim 9, wherein thealloy comprises, in % by weight: from about 0.9 to about 1.6 carbon fromabout 5 to about 13 manganese from about 2 to about 4 molybdenum from 0to about 4.5 boron from about 0.5 to about 1.5 silicon from about 1 toabout 3 copper from about 0.01 to about 4 cobalt from about 4 to about12.5 nickel plus cobalt.
 12. The alloy of claim 10, wherein the alloycomprises, in % by weight: from about 1 to about 1.55 carbon from about5 to about 12 manganese from about 2 to about 3.5 molybdenum from 0 toabout 4 boron from about 0.6 to about 1.2 silicon from about 1 to about2.5 copper from about 0.02 to about 4 cobalt from about 4 to about 12nickel plus cobalt.
 13. The alloy of claim 1, which exhibits a PREN offrom 58 to
 66. 14. The alloy of claim 11, which exhibits a PREN of from58 to
 66. 15. The alloy of claim 13, wherein the matrix comprises fromabout 0.25% to about 0.45% by weight of nitrogen in solid solution form.16. The alloy of claim 1, wherein the alloy comprises, in % by weight:from about 41 to about 48 chromium from about 0.45 to about 0.7 nitrogenfrom about 6 to about 30 manganese from about 0.9 to about 1.5 carbonfrom 0 to about 3.5 boron from 0 to about 4 molybdenum from 0 to about 3silicon from 0 to about 8 copper from 0 to about 25 nickel plus cobalt,the balance comprising iron and inevitable impurities.
 17. The alloy ofclaim 16, wherein the alloy comprises of at least one of molybdenum,silicon, boron, copper and (nickel plus cobalt), each in an amount of atleast about 0.01% by weight.
 18. The alloy of claim 17, wherein thealloy comprises, in % by weight, one or more of the following: fromabout 1 to about 4 molybdenum from about 0.5 to about 3 silicon fromabout 1 to about 8 copper from about 10 to about 25 nickel plus cobalt.19. The alloy of claim 18, wherein a PREN is from 51 to
 72. 20. Thealloy of claim 16, wherein the matrix comprises from about 0.25% toabout 0.45% by weight of nitrogen in solid solution form.
 21. A castingwhich comprises the alloy of claim
 1. 22. A casting which comprises thealloy of claim
 12. 23. A part of a slurry pump which comprises the alloyof claim
 1. 24. The part of claim 23, wherein the part comprises one ofa casing, impeller, suction liner, pipe, nozzle, agitator and a valveblade.